Image detail enhancement is frequently used in digital video systems such as digital television sets. A goal of image detail enhancement is to improve the image sharpness. As such, image high frequency components that contain image details are extracted, enhanced and added back to the original image so that the details in the processed image become more obvious to a viewer.
FIG. 1 shows a block diagram of a conventional image detail enhancement system 10, also known as unsharp masking. An original image f is passed through a low pass filter (LPF) 12 to obtain an image f1 (unsharp signal), wherein the image f1 is subtracted from the original image f in a node 14, to obtain the difference (f−f1). The difference (f−f1) is then boosted by a factor of K (K>0) in a multiplier 16, before being added back to the original image f in a node 18, to generate an enhanced output image g. The relationship between the output signal g and the input signal f can be expressed as:g=(f−f1)*K+f  (1)
The low pass filter 12 can be either a one dimensional (1D) filter or a two dimensional (2D) filter. If it is a 1D filter, the detail enhancement process can be performed along the horizontal and vertical directions separately.
Generally, an image edge can be enhanced in detail enhancement processes because an image edge is usually associated with high frequency image components. However, using a system shown in FIG. 1, some artifacts can also be introduced into the edge area.
An instance of such visual artifacts is a zigzagged edge due to conventional image detail enhancement. An example of the zigzagged edge artifact is described in conjunction with FIGS. 2A-B. FIG. 2A shows an original image edge 20, wherein each small square block 21 in the edge 20 represents one image pixel. The edge direction has a low angle relative to the horizontal direction. Along the horizontal direction across the edge, there is a long luminance transition range 22 from dark area to bright area or vice versa, as indicated in FIG. 2A. The luminance transition range 22 refers to the length of the luminance transitioning area of an edge either along the horizontal direction or along the vertical direction across the edge. With such a luminance transitioning area around the edge, the boundary of the edge shown in FIG. 2A looks generally smooth even though the edge has a limited image resolution.
A conventional image detail enhancement process is applied to the image edge 20 of FIG. 2A to generate the enhanced image 24 in FIG. 2B. Because in an image detail enhancement process high frequency components are boosted, the luminance transition range 22 may become shorter (or sharper). As can be seen in FIG. 2B, the luminance transition range 22 along the horizontal direction has become much shorter relative to that in FIG. 2A. As a consequence, the edge boundary now looks zigzagged. The more the image is enhanced in the detail enhancement process, the more obvious this kind of artifact would be. As a result, even though the image in FIG. 2B is enhanced, the quality of the image looks poor due to the degradation of edge quality.
The problem shown in FIG. 2B exists for most slant image edges. A slant image edge refers to an image edge whose direction is not exactly vertical or horizontal. Only when an image edge has exactly vertical or horizontal direction or precisely ±45° (i.e., +45° or −45°) direction, it is immune to the problem shown in FIG. 2B. Otherwise, a slant image edge can develop zigzagged edge artifacts if it is enhanced substantially.
As such, there is a need for a method of preserving edge quality and preventing zigzagged edge artifacts in detail enhancement, without sacrificing the enhancement of other image details that are not prone to zigzagged edge artifacts when enhanced.